1. Field of the Invention
The present invention relates to digital communications systems, and more particularly, to the processing of signalling information that is passed between facilities of a telecommunications system. The invention provides a multi-task signalling translator for translating signalling information between system components having incompatible signal coding schemes.
2. Description of the Prior Art
Digital switch interfaces or digital channel banks are generally used as voice terminals for a digital transmission network. These devices typically interface a conventional central office switch and a digital transmission facility. They sample the speech amplitude and the signalling state of each of a plurality of trunks or voice channels in rotation. The sampled information is multiplexed on a time division basis and is transmitted directly to a digital line, or by one or more stages of digital multiplexing, to a higher speed digital line. At the receiving end, which may be another central office switch or a digital remote terminal, another digital interface or channel bank restores the original transmission amplitudes and signalling states.
Thus, the role of the digital channel banks and digital interfaces is to reproduce the voice and signalling information so that from the point of view of the central office and or remote terminal, they appear to be interconnected by metallic wire.
Signalling continuity is maintained by detecting the incoming signalling state of each trunk at each channel unit of a digital channel bank or digital switch interface and periodically a digital line bit carries this detected information to the receive terminal or channel bank and then to the receive channel unit.
In North America, most telephony transmission is pulse code modulated based on the DS1, 24 channel time division multiplex scheme or very similar 24 channel time division formats. Each of the channel periods contain 8 PCM coded bits resulting in 192 bits for 24 channels. An additional bit called the frame bit is added to distinguish the beginning and end of the repeated 24 channel format. The 193 bits are referred to as a frame and are transmitted at a 1.544 Mbps rate. FIG. 1 shows the DSl format.
Signalling for functions such as ringing and call origination or detection has been accomplished by coding two signal bits, A and B, which are inserted into each of the 24 channels once every twelve frames. The frame bit is coded to identify a repeating group of 12 frames referred to as a superframe. In frame 6, the least significant bit of each 8-bit PCM channel word is replaced with the state of the channel's A signal bit. In frame 12, the least significant bit in each channel is replaced with the channel's B signal bit.
In modern telephony, many different types of services are offered. The most common is POTS, plain old telephone service. Other common services are: 4 WEM (4-wire E&M signalling), COIN, 2-party divided, bridge frequency selective and ground start. All of these services use the A/B signalling in PCM transmission. In providing these services, equipment manufacturers have implemented different coding schemes for the A/B signal bits, which have evolved over the years. A good example of this is illustrated by products based on the TR-TSY-000008 Digital Interface Specification (TR8) for digital interfaces between a SLC 96 Digital Loop Carrier System using the DS1 format and a local digital switch. Other products are based on the PUB 43801 (D4) specification for digital channel banks associated with a DS1 formatted digital loop carrier.
The TR-TSY-000008 (TR8) is a recent specification from Bell Communications Research and sets forth the interface requirements for direct connection of a digital loop carrier to a central office switch. The interface requirements were specifically based on AT&T/Western Electric's SLC 96 Digital Loop Carrier and the No.5 ESS Central Office Switch. With the publication of these interface requirements, other manufacturers of existing central office switches and digital loop carrier products can design their products to be compatible with this common interface specification.
To make existing equipment compatible with the interface requirements, the coding scheme for the A/B signal bits of the existing equipment must be translated in a real-time fashion to the coding scheme used by the interface specification. This signalling translation function will be a requirement for all future interface standards.
Given the multitude of signalling schemes and types of telephone service available, small changes or differences in signalling schemes for telephone services, between equipment manufacturers, can create expensive and time consuming hardware changes if translators are not programmable. Thus, there exists a need for a signalling translator that is programmable and that can accommodate the various types of telephone service presently offered and that may be offered in the future.